Lung metastasis in four experimental model systems was not affected by mouse age, and liver metastases were unaffected in one model system. In three brain metastasis model systems, young mice developed significantly greater metastases. Current efforts are examining (1) other brain metastasis models, (2) the role of the young immune system in producing increased metastases, (3) proteomic differences in the metastatic microenvironment of young and aged brains. With regard to immunity, collaborative experiments have shown an immunosuppressive infiltration of T- and monocytic cells into brain metastasis containing animals. Select subsets of immune cells vary by mouse age in metastatic brains, and their functional contributions are undergoing testing.